It is a usual practice to spot-welding together two electrogalvanized steel sheets placed one on top of the other. The spot welding comprises pinching two electrogalvanized steel sheets placed one on top of the other between a pair of electrode tips made of copper, and electrifying the pair of electrode tips while pressing the electrogalvanized steel sheets against each other by means of the pair of electrode tips, to electric-resistance-welding together these two electrogalvanized steel sheets.
When spot-welding together two electrogalvanized steel sheets, the following problem is encountered: The galvanizing layers of the weld zone of the two electrogalvanized steel sheets are melted by the welding heat generated upon the spot welding. The resultant molten zinc of the galvanizing layers reacts with copper contained in the pair of electrode tips to produce a hard and brittle zinc-copper alloy layer on each of the pair of electrode tips. This results in a violent wear of the electrode tips, thus leading to a lower spot weldability including a shorter service life of the electrode tips and a impaired electric conductivity thereof.
As a method for solving the above-mentioned problem and manufacturing an electrogalvanized steel sheet excellent in spot weldability, the following method is proposed:
A method for manufacturing an electrogalvanized steel sheet excellent in spot weldability, as disclosed in Japanese Patent Provisional Publication No. 1-205,090 dated Aug. 17, 1989, which comprises the steps of: electrogalvanizing a steel sheet in an acidic electrogalvanizing solution containing hydrogen peroxide in an amount within a range of from 0.5 to 30 % to form, on at least one surface of said steel sheet, a galvanizing layer comprising zinc oxide or zinc hydroxide, in an amount within a range of from 0.03 to 3.0 g/m.sup.2 per surface of said steel sheet (hereinafter referred to as the "Prior Art").
According to the above-mentioned Prior Art, it is possible to manufacture an electrogalvanized steel sheet excellent in spot weldability, having, on at least one surface thereof, a galvanizing layer comprising zinc oxide or zinc hydroxide. The galvanizing layer comprising zinc oxide or zinc hydroxide hardly reacts with copper. When spot-welding, therefore, almost no zinc-copper alloy layer is produced on each of the pair of electrode tips, through the reaction of zinc with copper, thus inhibiting wear of the electrode tips. Furthermore, the galvanizing layer comprising zinc oxide or zinc hydroxide is high in melting point and electric resistance. The galvanizing layer comprising zinc oxide or zinc hydroxide is therefore excellent in spot weldability.
When electrogalvanizing a steel sheet in an acidic electrogalvanizing solution containing hydrogen peroxide as in the Prior Art, a galvanizing layer comprising zinc oxide or zinc hydroxide is formed on at least one surface of the steel sheet, and the reason of this is estimated to be as follows:
When electrogalvanizing a steel sheet in an acidic electrogalvanizing solution containing hydrogen peroxide as an oxidizer, the reduction reaction of zinc ions causes the increase to 5.6 in the pH value of the acidic electrogalvanizing solution on the interface of the cathode, i.e., the steel sheet, and the reduction reaction of hydrogen peroxide as the oxidizer further increases the above-mentioned pH value to over 5.6. This increase in the pH value of the electrogalvanizing solution on the interface of the cathode causes the precipitation of zinc oxide or zinc hydroxide on the surface of the steel sheet as the cathode. A galvanizing layer comprising zinc oxide or zinc hydroxide can thus be formed on at least one surface of the steel sheet.
However, the above-mentioned Prior Art has the following problems: As described above, the pH value of the electrogalvanizing solution on the interface of the cathode, i.e., the steel sheet increases to over 5.6 under the effect of the reduction reaction of the oxidizer contained in the acidic electrogalvanizing solution. As a result, there is an increase in the weight of the galvanizing layer comprising zinc oxide or zinc hydroxide formed on at least one surface of the steel sheet. However, if the weight of the thus formed galvanizing layer comprising zinc oxide or zinc hydroxide becomes excessive, the welding current becomes difficult to flow upon application of the spot welding. This leads to a lower spot weldability of the electrogalvanized steel sheet, and moreover, to a poorer workability thereof. According to the present inventors' research, the weight of the galvanizing layer comprising zinc oxide or zinc hydroxide, which is capable of improving spot weldability, is within a range of from 0.05 to 1.00 g/m.sup.2 per surface of the steel sheet.
The content of the oxidizer in the acidic electrogalvanizing solution sensively affects the pH value of the electrogalvanizing solution on the interface of the cathode. When the pH value of the electrogalvanizing solution on the interface of the cathode decreases to 5.6 or under, the weight of the galvanizing layer comprising zinc oxide or zinc hydroxide decreases to under 0.05 g/m.sup.2 per surface of the steel sheet, thus degrading spot weldability. When the above-mentioned pH value increases to over 12, on the other hand, the weight of the above-mentioned galvanizing layer increases to over 1.0 g/m.sup.2 per surface of the steel sheet, thus also degrading spot weldability. The range of the content of the oxidizer, which is capable of improving spot weldability of the electrogalvanized steel sheet, is therefore very narrow.
Upon electrogalvanizing a steel sheet, it is very difficult to keep the content of the oxidizer in the acidic electrogalvanizing solution within a narrow range which can improve spot weldability. It is therefore very difficult to stably manufacture an electrogalvanized steel sheet excellent in spot weldability, having on at least one surface thereof, a galvanizing layer comprising zinc oxide or zinc hydroxide, in an amount appropriate for improving spot weldability.
Under such circumstances, there is a strong demand for the development of a method for stably manufacturing an electrogalvanized steel sheet excellent in spot weldability, which has on at least one surface thereof a galvanizing layer comprising zinc oxide or zinc hydroxide, in an amount appropriate for improving spot weldability, but such a method has not as yet been proposed.